2,139 research outputs found
Analysis of the Early-time Optical Spectra of SN 2011fe in M101
The nearby Type Ia supernova (SN Ia) SN 2011fe in M101 (cz = 241 km s^(–1)) provides a unique opportunity to study the early evolution of a "normal" SN Ia, its compositional structure, and its elusive progenitor system. We present 18 high signal-to-noise spectra of SN 2011fe during its first month beginning 1.2 days post-explosion and with an average cadence of 1.8 days. This gives a clear picture of how various line-forming species are distributed within the outer layers of the ejecta, including that of unburned material (C+O). We follow the evolution of C II absorption features until they diminish near maximum light, showing overlapping regions of burned and unburned material between ejection velocities of 10,000 and 16,000 km s^(–1). This supports the notion that incomplete burning, in addition to progenitor scenarios, is a relevant source of spectroscopic diversity among SNe Ia. The observed evolution of the highly Doppler-shifted O I λ7774 absorption features detected within 5 days post-explosion indicates the presence of O I with expansion velocities from 11,500 to 21,000 km s^(–1). The fact that some O I is present above C II suggests that SN 2011fe may have had an appreciable amount of unburned oxygen within the outer layers of the ejecta
Photometric variability of candidate white dwarf binary systems from Palomar Transient Factory archival data
We present a sample of 59 periodic variables from the Palomar Transient
Factory, selected from published catalogues of white dwarf (WD) candidates. The
variability can likely be attributed to ellipsoidal variation of the tidally
distorted companion induced by the gravity of the primary (WD or hot subdwarf)
or to the reflection of hot emission by a cooler companion. We searched 11311
spectroscopically or photometrically selected WD candidates from three hot
star/WD catalogues, using the Lomb-Scargle periodogram to single out promising
sources. We present period estimates for the candidates, 45 of which were not
previously identified as periodic variables, and find that most have a period
shorter than a few days. Additionally, we discuss the eclipsing systems in our
sample and present spectroscopic data on selected sources
Weak Prezygotic Isolating Mechanisms in Threatened Caribbean Acropora Corals
The Caribbean corals, Acropora palmata and A. cervicornis, recently have undergone drastic declines primarily as a result of disease. Previous molecular studies have demonstrated that these species form a hybrid (A. prolifera) that varies in abundance throughout the range of the parental distribution. There is variable unidirectional introgression across loci and sites of A. palmata genes flowing into A. cervicornis. Here we examine the efficacy of prezygotic reproductive isolating mechanisms within these corals including spawning times and choice and no-choice fertilization crosses. We show that these species have subtly different mean but overlapping spawning times, suggesting that temporal isolation is likely not an effective barrier to hybridization. We found species-specific differences in gametic incompatibilities. Acropora palmata eggs were relatively resistant to hybridization, especially when conspecific sperm are available to outcompete heterospecific sperm. Acropora cervicornis eggs demonstrated no evidence for gametic incompatibility and no evidence of reduced viability after aging four hours. This asymmetry in compatibility matches previous genetic data on unidirectional introgression
Phase resolved spectroscopy and Kepler photometry of the ultracompact AM CVn binary SDSS J190817.07+394036.4
{\it Kepler} satellite photometry and phase-resolved spectroscopy of the
ultracompact AM CVn type binary SDSS J190817.07+394036.4 are presented. The
average spectra reveal a variety of weak metal lines of different species,
including silicon, sulphur and magnesium as well as many lines of nitrogen,
beside the strong absorption lines of neutral helium. The phase-folded spectra
and the Doppler tomograms reveal an S-wave in emission in the core of the He I
4471 \AA\,absorption line at a period of \,sec
identifying this as the orbital period of the system. The Si II, Mg II and the
core of some He I lines show an S-wave in absorption with a phase offset of
compared to the S-wave in emission. The N II, Si III and some
helium lines do not show any phase variability at all. The spectroscopic
orbital period is in excellent agreement with a period at \,sec detected in the three year {\it Kepler} lightcurve. A
Fourier analysis of the Q6 to Q17 short cadence data obtained by {\it Kepler}
revealed a large number of frequencies above the noise level where the majority
shows a large variability in frequency and amplitude. In an O-C analysis we
measured a xs\,s for some of
the strongest variations and set a limit for the orbital period to be
s\,s. The shape of the phase folded
lightcurve on the orbital period indicates the motion of the bright spot.
Models of the system were constructed to see whether the phases of the radial
velocity curves and the lightcurve variation can be combined to a coherent
picture. However, from the measured phases neither the absorption nor the
emission can be explained to originate in the bright spot.Comment: Accepted for publication in MNRAS, 15 pages, 14 figures, 5 table
Statistical Searches for Microlensing Events in Large, Non-Uniformly Sampled Time-Domain Surveys: A Test Using Palomar Transient Factory Data
Many photometric time-domain surveys are driven by specific goals, such as
searches for supernovae or transiting exoplanets, which set the cadence with
which fields are re-imaged. In the case of the Palomar Transient Factory (PTF),
several sub-surveys are conducted in parallel, leading to non-uniform sampling
over its footprint. While the median PTF field has been imaged 40 times in \textit{R}-band,
have been observed 100 times. We use PTF data to
study the trade-off between searching for microlensing events in a survey whose
footprint is much larger than that of typical microlensing searches, but with
far-from-optimal time sampling. To examine the probability that microlensing
events can be recovered in these data, we test statistics used on uniformly
sampled data to identify variables and transients. We find that the von Neumann
ratio performs best for identifying simulated microlensing events in our data.
We develop a selection method using this statistic and apply it to data from
fields with 10 -band observations, light curves,
uncovering three candidate microlensing events. We lack simultaneous,
multi-color photometry to confirm these as microlensing events. However, their
number is consistent with predictions for the event rate in the PTF footprint
over the survey's three years of operations, as estimated from near-field
microlensing models. This work can help constrain all-sky event rate
predictions and tests microlensing signal recovery in large data sets, which
will be useful to future time-domain surveys, such as that planned with the
Large Synoptic Survey Telescope.Comment: 13 pages, 14 figures; accepted for publication in ApJ. fixed author
lis
Nature-Inspired Interconnects for Self-Assembled Large-Scale Network-on-Chip Designs
Future nano-scale electronics built up from an Avogadro number of components
needs efficient, highly scalable, and robust means of communication in order to
be competitive with traditional silicon approaches. In recent years, the
Networks-on-Chip (NoC) paradigm emerged as a promising solution to interconnect
challenges in silicon-based electronics. Current NoC architectures are either
highly regular or fully customized, both of which represent implausible
assumptions for emerging bottom-up self-assembled molecular electronics that
are generally assumed to have a high degree of irregularity and imperfection.
Here, we pragmatically and experimentally investigate important design
trade-offs and properties of an irregular, abstract, yet physically plausible
3D small-world interconnect fabric that is inspired by modern network-on-chip
paradigms. We vary the framework's key parameters, such as the connectivity,
the number of switch nodes, the distribution of long- versus short-range
connections, and measure the network's relevant communication characteristics.
We further explore the robustness against link failures and the ability and
efficiency to solve a simple toy problem, the synchronization task. The results
confirm that (1) computation in irregular assemblies is a promising and
disruptive computing paradigm for self-assembled nano-scale electronics and (2)
that 3D small-world interconnect fabrics with a power-law decaying distribution
of shortcut lengths are physically plausible and have major advantages over
local 2D and 3D regular topologies
Convergence of expansions in Schr\"odinger and Dirac eigenfunctions, with an application to the R-matrix theory
Expansion of a wave function in a basis of eigenfunctions of a differential
eigenvalue problem lies at the heart of the R-matrix methods for both the
Schr\"odinger and Dirac particles. A central issue that should be carefully
analyzed when functional series are applied is their convergence. In the
present paper, we study the properties of the eigenfunction expansions
appearing in nonrelativistic and relativistic -matrix theories. In
particular, we confirm the findings of Rosenthal [J. Phys. G: Nucl. Phys. 13,
491 (1987)] and Szmytkowski and Hinze [J. Phys. B: At. Mol. Opt. Phys. 29, 761
(1996); J. Phys. A: Math. Gen. 29, 6125 (1996)] that in the most popular
formulation of the R-matrix theory for Dirac particles, the functional series
fails to converge to a claimed limit.Comment: Revised version, accepted for publication in Journal of Mathematical
Physics, 21 pages, 1 figur
Solution of the Fokker-Planck equation with a logarithmic potential and mixed eigenvalue spectrum
Motivated by a problem in climate dynamics, we investigate the solution of a
Bessel-like process with negative constant drift, described by a Fokker-Planck
equation with a potential V(x) = - [b \ln(x) + a\, x], for b>0 and a<0. The
problem belongs to a family of Fokker-Planck equations with logarithmic
potentials closely related to the Bessel process, that has been extensively
studied for its applications in physics, biology and finance. The Bessel-like
process we consider can be solved by seeking solutions through an expansion
into a complete set of eigenfunctions. The associated imaginary-time
Schroedinger equation exhibits a mix of discrete and continuous eigenvalue
spectra, corresponding to the quantum Coulomb potential describing the bound
states of the hydrogen atom. We present a technique to evaluate the
normalization factor of the continuous spectrum of eigenfunctions that relies
solely upon their asymptotic behavior. We demonstrate the technique by solving
the Brownian motion problem and the Bessel process both with a negative
constant drift. We conclude with a comparison with other analytical methods and
with numerical solutions.Comment: 21 pages, 8 figure
Two Distant Halo Velocity Groups Discovered by the Palomar Transient Factory
We report the discovery of two new halo velocity groups (Cancer groups A and B) traced by 8 distant RR Lyrae stars and observed by the Palomar Transient Factory (PTF) survey at R.A.~129°, Dec~20° (l~205°, b~32°). Located at 92 kpc from the Galactic center (86 kpc from the Sun), these are some of the most distant substructures in the Galactic halo known to date. Follow-up spectroscopic observations with the Palomar Observatory 5.1-m Hale telescope and W. M. Keck Observatory 10-m Keck I telescope indicate that the two groups are moving away from the Galaxy at v_(gsr) = 78.0+-5.6 km s^(-1) (Cancer group A) and v_(gsr) = 16.3+-7.1 km s^(-1) (Cancer group B). The groups have velocity dispersions of σ_(v_)gsr))=12.4+-5.0 km s^(-1) and σ _(v_(gsr))=14.9+-6.2 km s^(-1), and are spatially extended (about several kpc) making it very unlikely that they are bound systems, and are more likely to be debris of tidally disrupted dwarf galaxies or globular clusters. Both groups are metal-poor (median metallicities of [Fe/H]^A = -1.6 dex and [Fe/H]^B =-2.1 dex), and have a somewhat uncertain (due to small sample size) metallicity dispersion of ~0.4 dex, suggesting dwarf galaxies as progenitors. Two additional RR Lyrae stars with velocities consistent with those of the Cancer groups have been observed ~25 ° east, suggesting possible extension of the groups in that direction
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